1. Field of the Invention
The present invention relates to a low-voltage electromagnetic riveting apparatus and method, and more particularly to a method and apparatus for controlled and efficient low-voltage electromagnetic riveting.
2. Background Information
Riveting machines are well known and in wide use throughout the aerospace industry, as well as in other industries. Rivets provide the best known technique for fastening an aerodynamic skin to a frame to provide a strong, aerodynamically smooth surface. Rivets are also used in the interior structure of an aircraft, since they are the lightest and least expensive way of fastening structural components together.
One form of riveting uses a low voltage electromagnetic riveting (LVEMR) system 100, as shown in FIG. 1A. The LVEMR system 100 provides a controlled amount of energy in a single pulse and is typically smaller and less cumbersome than a pneumatic or hydraulic system. Further, the LVEMR system has almost no mass so it only has nominal reactionary forces. The LVEMR system 100 shown in FIG. 1A incorporates two electromagnetic actuators, a first actuator 101 and a second actuator 112, which are positioned on opposite sides of first and second workpieces 114 and 115, respectively. The first and second work pieces 114 and 115 are sandwiched together and a hole has been drilled through them to accommodate a rivet 93. The first and second actuators 101 and 112 each include a body 116 in which is positioned a driver 118 and a coil 120. A rivet die 92 is coupled to the driver 118 and is forced against the rivet 93. Also, there may be a recoil mass 123 which is typically secured to a rear surface of the coil 120. Extending from the recoil mass 123 is an air cylinder rod 124, which extends out of the body 116 into a two-chamber air cylinder 126. Associated pressure relief valves and other control elements are shown diagramatically as block 128. The elements of block 128 are responsible for initially positioning the driver 118 and its rivet die 92 against a head of the rivet 93.
Power is supplied to the system 100 by means of a power supply 130. A DC output from the supply 130 is used to charge a bank of capacitors in circuit 132 to a selected voltage. The voltage selected is based on the force necessary to accomplish the desired riveting task. The circuit 132 includes an electronic switch positioned between the capacitors and the coil 120.
A trigger signal from a firing circuit 134 activates the electronic switch, dumping the charge of the capacitor bank in circuit 132 into the coil 120. A current pulse is induced into the coil 120 causing strong eddy currents in a copper plate 119 located at the base of the driver 118. This creates a very strong magnetic field that provides a repulsive force relative to the coil 120. The driver 118 is propelled forward with a large force causing the rivet die 92 to upset the head of the rivet 93. A more detailed discussion of low voltage electromagnetic riveting can be found in U.S. Pat. No. 4,862,043, which is incorporated herein by reference.
Once the LVEMR system 100 has upset the rivet 93, a fastened assembly 140 is created as shown in FIG. 1B. The assembly 140 includes a deformed rivet 146, having a head 142 and a tail 154. The hole drilled into the first and second workpieces 114 and 115 includes a countersink 148 drilled into the second workpiece 115 to receive the head 142 of the deformed rivet 146.
Unfortunately, the fastened assembly 140, when produced by the LVEMR system 100 described above, has significant gaps 150 between the head 142 of the deformed rivet 146 and the countersink 148. The gaps 150 are undesirable since they could lead to early corrosion of the deformed rivet 146, causing it to weaken and prematurely fail. Accordingly, for the foregoing reasons, there is a need in the art for a controlled low-voltage electromagnetic riveting apparatus and process that mitigates the gaps 150 between the rivet head 142 and the countersink 148.
In one aspect, the present invention is directed to a method for minimizing undesirable gaps in riveted assemblies including the steps of selecting a rivet having a head and a tail with identical forming characteristics, positioning the selected rivet in an assembly that is countersunk on one of two sides, and applying a force over time to the head of the rivet and a force over time to the tail of the rivet that are equal and opposite, compensating for force-unbalancing characteristics of the countersink.
In another aspect, the present invention is directed to a method for mitigating gaps between a deformed head of a rivet and a countersink in an assembly that is coupled by a low-voltage electromagnetic riveter having a head side actuator and tail side actuator. The method includes the steps of selecting a rivet that uniformly deforms at a tail and at a head of the rivet, positioning the volume of the rivet within the assembly such that force applied over time to the head of the rivet by the head side actuator equals a force applied over time to the tail of the rivet by the tail-side actuator.
In yet another aspect, the present invention is directed to a method for mitigating gaps between a head of a rivet and a countersink within a first workpiece of two workpieces when the rivet is upset by a low voltage electromagnetic riveting process. The method includes the steps of extending a tail of the rivet out of a surface of a second workpiece of the two workpieces by a length from 0.9 to 1⅓ times a diameter of the rivet, extending the head of the rivet out of a base of the countersink by a length that is 5% to 10% less than the length the tail of the rivet was extended out of the second workpiece surface, and upsetting the tail of the rivet with a tail die having a shape substantially similar to a shape of the countersink within the first workpiece.
In still another aspect, the present invention is directed to a method for controlled low-voltage electromagnetic riveting of a primary workpiece including a countersink and at least a secondary workpiece with a rivet, having a head, a tail, and a diameter, using a head actuator having a head die to contract the head of the rivet and a tail actuator having a tail die to contact the tail of the rivet. The method includes the steps of selecting the rivet so the rivet is comprised of a homogenous alloy and the rivet has a uniform diameter, positioning the tail of the rivet so that it protrudes from an outside surface of the secondary workpiece by a length from 1 to 1.3 times the diameter of the rivet, positioning the head of the rivet so that it protrudes from the base of the countersink by a length that is 5 to 10 percent less than the length that the tail protrudes from the step of positioning the tail, upsetting the head of the rivet with the head die having a flat contact surface, and upsetting the tail of the rivet with the tail die, wherein the tail die has an upper diameter within 20% of the depth of the countersink, and wherein the tail die has an upper diameter within 10 degrees of the upper angle of the countersink.
In another aspect, the present invention is directed to a low-voltage electromagnetic riveter for controlling the force over time applied to a head and a tail of a rivet within an assembly having a workpiece that is countersunk to receive the head of the rivet. The riveter includes a head and a tail actuator that respectively apply a force over time to the head and the tail of the rivet. Each of the actuators includes a die which contacts the rivet, a coil which creates a repulsive force when electrical current is passed therethrough, a driver physically adjacent to the coil and movable along an axis of the rivet by the repulsive force created by the coil, and a load cell positioned between the driver and the die to measure the force over time applied to a designated end of the rivet. A head current source and a tail current source are electrically connected to the coil of the respective head and tail actuator for supplying a controlled amount of current, and a firing circuit is electrically connected to each of the head current source and the tail current source for controlling phase and magnitude of the controlled amount of current supplied to each of the head actuator and the tail actuator.
In yet another aspect, the present invention is directed to a method for controlled low-voltage electromagnetic riveting. The method includes the steps of monitoring the force applied over time to a head and tail of a rivet during a deformation of the rivet by the low-voltage electromagnetic riveting, adjusting a phase of the force applied to at least one of a location of the head and the tail of the rivet so that the phase of the force applied to the location of the head of the rivet equals the phase of the force applied to the location of the tail of the rivet, and adjusting a magnitude of the force applied to at least one of the location of the head and the tail of the rivet so that the magnitude of the force applied to the location of the rivet head equals the force applied to the location of the tail of the rivet.
In still another aspect, the present invention is directed to a method for mitigating gaps between a deformed head of a rivet and a countersink in an assembly that is coupled by a low-voltage electromagnetic riveter, including a head-side driver, having a first load cell, and a tail side driver, having a second load cell, and a firing control circuit capable of controlling phase and magnitude of force applied by the head-side driver and the tail-side driver. The method includes the steps of positioning a first test rivet within the assembly, monitoring a first output of the first load cell and the second load cell while the first test rivet is upset to determine the phase and the magnitude of the force applied to a head and a tail of the rivet respectively by the head side driver and the tail side driver, comparing the first output of the first load cell and the second load cell that occurred when the first test rivet was upset, and adjusting the phase of one of the force applied by the head driver and the force applied by the tail driver so that the phase of the force applied by the head driver matches the phase of the force applied by the tail driver. The method also includes the steps of positioning a second test rivet within the assembly, monitoring a second output of the first load cell and the second load cell while the second test rivet is upset to determine the phase and the magnitude of the force applied to the head and the tail of the second test rivet respectively by the head side driver and the tail side driver, comparing the second output of the first load cell and the second load cell that occurred when the second test rivet was upset, and adjusting the magnitude of one of the force applied by the head driver and the force applied by the tail driver so that the magnitude of the force applied by the tail driver equals the magnitude of the force applied by the head driver.